The user will be added to the system and his home dir created.
To remove a user from the system, run the deluser command as root with the user name.
The user will be removed from the system. To delete his home directory add --remove-home option.
To remove all user files and home dir in one hit, add --remove-all-files option. This will remove all mail files also.
To add a user to a group, run the adduser command with user and group name. Use no options:
adduser obama cdrom
Adds user obama to the group cdrom.
To remove a user from a group run deluser command with no options:
deluser obama cdrom
Removes user obama from the group cdrom.
To add a new group to the system run the addgroup command as root.
To delete a group from the system run the delgroup command as root.
There are another set of commands for adding users and groups:
useradd / groupadd
They operate a little different to adduser addgroup.
Check out the man page for more info.
That's about it!